The invention relates generally to a seat belt retractor for a vehicle occupant restraint system and more particularly to a winding prevention mechanism for a belt retractor wherein the torsional capacity of the belt windup spring is minimized by an operator actuatable device for selectively terminating the frictional impediment which the winding prevention mechanism normally exerts in opposition to the belt winding effort exerted by the windup spring.
Occupant restraint systems for a motor vehicle commonly employ a shoulder belt which extends over the shoulder and across the chest of the seated occupant. One type of well known seat belt system is a three-point belt system comprising a single length of belt having its lower end mounted to the vehicle floor outboard the occupant seating position and its upper end attached to the vehicle body by an inertia responsive seat belt retractor. A latch plate assembly is mounted on the belt intermediate the ends and is engageable with a buckle mounted inboard of the occupant seating position to divide the belt into a shoulder belt portion and a lap belt portion. The seat belt retractor utilized in such a seat belt system commonly utilizes a relatively strong windup spring which rotates a reel to wind a substantial portion of the shoulder belt onto the reel and suspend the latch plate assembly and lap belt portion in a generally vertical stored position adjacent the wall of the passenger compartment. It is known that three-point belt system are effective for restraint of the occupant even though the shoulder belt portion is in a somewhat slackened relation with respect to the chest of the occupant.
It has been discovered that utilization of a relatively strong windup spring in the retractor results in a level of shoulder belt load across the chest which is annoying to some seat occupants. It is therefore desirable to provide means for relieving the tension of the windup spring on the belt so as to relieve the belt tension across the chest of the occupant.
It is known to provide a restraint belt retractor having a winding prevention mechanism which selectively relieves the tension on the belt response to a predetermined sequence of belt movement by the restrained occupant. U.S. Pat. No. 3,869,098 by Raymond G. Sprecher, issued Mar. 4, 1975, and assigned to the assignee of this invention, discloses a belt retractor of the aforedescribed type wherein a control disc and a blocking disc are frictionally clutched to the reel for rotation therewith and cooperate to control movement of a winding prevention pawl into detenting engagement with ratchet teeth carried by the reel. Rotation limiting devices acting between the discs and the retractor housing define limits at which rotation of the discs are stopped whereupon the friction clutch acting between the discs and the reel begins to slip to permit continued rotation of the reel while the discs remain stationary.
U.S. Pat. No. 3,973,786, by Lloyd W. Rogers, Jr., filed Mar. 28, 1975 and assigned to the assignee of this invention, discloses a seat belt retractor wherein a winding prevention pawl is moved to an undetenting position in response to opening movement of the vehicle door so that the winding prevention mechanism is automatically cancelled by disengaging the pawl from the ratchet teeth to wind the belt whenever the door is moved from closed to open position. This assures retraction of the belt system to stored position to facilitate ease of occupant ingress and egress independent of the occupant's performance of the sequence of belt movement for cancelling the winding prevention mechanism.
A disadvantage of the prior known winding prevention mechanisms is that the control mechanism for operating the winding prevention pawl imparts a frictional force to the reel which opposes belt winding reel rotation and windup of the belt by the windup spring. Accordingly, the torsional capacity of the windup spring must be sufficient to overcome the frictional impedance imparted to the reel by the control mechanism. For example, in the winding prevention mechanism of the aforedescribed Sprecher and Rogers patents, the windup spring must have a sufficient torsional capacity to overcome the frictional impedance of the friction clutch which acts between the discs and the reel.